Sand table v3: build fixes, bolted hub, counterweight, print guide

- print/ reorganised: v1-original (first export), v2-current, the v2.1-v2.7 fixes
  found while building, and v3/ with every printed part.
- v3: deck screws you can reach, seam joints from outside, arm bolted straight
  to the hub, 6804 hub bearings with a 15 mm bore for connectors, clamp collar,
  counterweight, 12.5 mm sand ring for a raised floor, Uno turnaround plate.
- print/README.md: full v3 parts list, what changed and why, assembly order.
- docs/: diagrams for each change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,6 @@
 .DS_Store
 *.swp
 *~
+
+# Python
+__pycache__/